This invention relates to metal-air cells, and to a power system using metal-air cells.
Metal-air cells are well-known primary cells having an anode of a reactive metal, such as aluminum or magnesium, and an air cathode spaced in close proximity to, but not touching, the anode. A suitable electrolyte is circulated through the cell to electrochemically couple the anode and cathode, releasing electrons and creating a potential that results in the flow of current when the cell is connected across a load. During these reactions, the anode is consumed.
Several difficulties have interfered with the practical application of metal air cells in large power systems. One of these difficulties is in "refueling" or replenishing the cell after the anode is consumed. In the past, the refueling process was time consuming, and often resulted in substantial down time. Another of these difficulties was the problem of cell voltage degradation as the anode was consumed. Consumption of the anode caused the anode-cathode spacing in the cell to change. Moreover, in the extreme, consumption of the anode caused a loss of structural and electrical integrity. Another difficulty was degradation of the electrolyte solution. As the reaction in the cell proceeded, reaction products built up in the electrolyte solution, and concentration of the electrolyte decreased, both of which caused a decrease in performance of the cell. Still another difficulty with metal-air cells has been slow start up and difficulty in turning the cells off. Optimum operating electrolyte temperature for most metal air cells is relatively high, typically about 130.degree. F. to 150.degree. F. Metal-air cells generate low power at low temperatures, and thus in prior art it could take several minutes before cell operation would warm up the circulating electrolyte to a satisfactory operating temperature. Moreover, once the cell was turned on it was difficult to turn off the cell and prevent further consumption of the anode to preserve the life of the cell.
Generally, the metal air cell of the present invention comprises a flexible, recloseable, pouch made of a gas-permeable, electrolyte-impermeable, material which forms the cathode of the cell. There is a metal plate anode in the pouch, and spacers physically isolating the anode from the interior of the flexible pouch cathode, separating the anode and the cathode by a predetermined spacing. The pouch is preferably formed from two panels of a gas-permeable, electrolyte-impermeable, material joined together at their respective bottom and side edges to form a pouch, open at the top to provide access to the pouch. There are elongated, resilient sealing beads on each panel along each side of the opening of the pouch. A clamp releasably compresses the sealing beads together to close and seal the top edges of the panels to close the pouch. This recloseable opening in the pouch cathode allows the anode to be conveniently replaced after it has been consumed. The pouch has an inlet and an outlet for circulation of an electrolyte through the cell. The electrolyte may be a solution of KOH, NaOH, or NaCl, or some other suitable material.
A plurality of such cells can be stacked to form a multi-cell battery. The stack is preferably surrounded by a harness with a spring for tensioning the harness to compress the cells to maintain the predetermined anode-cathode spacing determined by the spacers in the cell, as the anode in each cell is consumed. There are preferably air gap spacers between each cell in the stack for separating the individual cells with an air gap. These spacers are sized so that the size of the air gap between the cells may vary from cell to cell to provide variable cooling of the cells.
The anode plate is preferably a substantially flat metal plate with a highly conductive terminal extending therefrom, and may have a raised dendritic pattern thereon. The dendritic pattern comprises a tapering main stem that starts at the conductive terminal and extends substantially across the plate, and a plurality of tapering branches extending from the main stem. This pattern provides additional structural integrity and electrical communication across the anode plate to the terminal, even as the metal in the plate is consumed.
The power system of this invention preferably comprises a stack of such metal-air cells and a circulatory system for delivering an electrolyte solution to the inlets of the cells, and removing the electrolyte solution and chemical reaction products from the outlets of the cells. The circulatory system includes a pump, and a controller for controlling the operation of the power system. The circulatory system preferably includes a sensor for monitoring the concentration of electrolyte in the electrolyte solution circulating in the circulatory system, and an injector, such as a solenoid-controlled valve, responsive to the sensor, for injecting additional electrolyte into the circulatory system when the concentration of electrolyte in the solution drops below a predetermined minimum. The sensor preferably monitors electrolyte concentration by monitoring the conductivity or pH of the electrolyte. The circulatory system may also include a sump with baffles for trapping solid particles that form in the electrolyte solution, and/or an electrolyte filter for removing particles from the circulating electrolyte solution.
The circulatory system preferably also comprises a heat exchanger through which the electrolyte solution can be circulated to lower its temperature, and a sensor for monitoring the temperature of the electrolyte solution circulating in the circulatory system. The controller can direct electrolyte solution through the heat exchanger when the temperature of the solution reaches a predetermined temperature. Alternatively, and preferably, the electrolyte solution can circulate continuously through the heat exchanger, and the system can further comprise a cooling fan for forcing cooling air over the heat exchanger, and the controller can simply operate the fan in response to the sensor for monitoring the temperature of the electrolyte solution circulating in the circulatory system.
The power system preferably also includes a supplemental battery, connected in parallel with the stack, to provide electrical power to start system operation and to provide electrical power throughout shut-down. The supplemental battery can be sized to provide supplementary power to the power system during periods of peak current demand, and for providing operating power for the system, including the pump, sensors, and controller.
The electrolyte solution is preferably initially stored in a sump connected to the circulatory system. There are preferably temperature sensors in at least one of the cells, and in the circulatory system. When the power system is turned on, the controller determines whether the system temperature exceeds a critical start temperature, T.sub.s. If the system temperature is greater than T.sub.s, the controller turns the circulatory system on to continuously circulate electrolyte; if the system temperature is less than T.sub.s, the controller initially turns on the circulatory system for a first predetermined period of time t.sub.1, to fill the cells with electrolyte solution, and then turns off the circulatory system for a second predetermined period of time t.sub.2. The controller then monitors the temperature of the electrolyte solution in at least one of the cells through the temperature sensor t.sub.1 in the cell. If the cell temperature is less than a predetermined minimum T.sub.1, the controller reactivates the circulatory system for the first predetermined time to exchange the electrolyte solution in the cells, and then turns off the circulatory system for the second predetermined period t.sub. 2, and repeats this pulse on and pulse off operation until the cell temperature exceeds the predetermined cell temperature T.sub.1. When the cell temperature exceeds T.sub.1 the controller then determines whether the system temperature exceeds T.sub.s. If the temperature does exceed T.sub.s, the circulatory system remains on continuously; if the temperature does not exceed T.sub.s, the control system resumes the pulse on and pulse off operation until the controller again determines that the cell temperature exceeds T.sub.1.
The controller also operates immediately after the power system is turned off to cause the circulatory system to draw the electrolyte from the cells, to thereby preserve the anodes. However, when the power system is initially turned off the controller preferably connects the supplemental battery across the stack with reverse polarity for a predetermined time to inhibit electron flow and so preserve the anodes in the cells. This holds the power system in a ready state, in case the power system is turned on again. If the power system is not turned on again within the predetermined time, the controller then causes the circulatory system to draw the electrolyte from the cells.
The stack is preferably provided as a separate, replaceable component of the system. Thus, when the anodes are expended, the stack can simply be replaced. The depleted stack can be replenished by opening each cell within the stack and installing new anodes.
This invention also relates to a method of starting up a power source comprising a plurality of metal-air cells and a circulatory system for circulating an electrolyte solution through the cells. Before start-up, the cells are substantially empty of electrolyte solution. Generally, the method comprises the steps of comparing the system temperature to a critical start temperature T.sub.s. If the system temperature exceeds the critical start temperature T.sub.s, the controller turns on the circulatory system to operate continuously to circulate the electrolyte solution. If the system temperature does not exceed the critical start temperature T.sub.s, the controller turns on the circulatory system for a first predetermined period of time t.sub.1 to fill the cells with electrolyte solution, and turns off the circulatory system for a second predetermined period of time t.sub.2. The controller then monitors the temperature of the electrolyte solution in at least one of the cells. If the cell temperature is less than a predetermined temperature T.sub.1, the controller turns on the circulatory system for the first predetermined time t.sub.1 to exchange the electrolyte solution in the cells, turns the circulatory system off for the second predetermined period of time t.sub.2, and repeats the pulse on/pulse off operation until the cell temperature exceeds the predetermined minimum T.sub.1. When the cell temperature exceeds T.sub.1, the controller determines whether the system temperature exceeds T.sub.s. If so the controller turns on the circulatory system for continuous operation; if not the controller continues the pulse on/pulse off operation of the circulatory system until the cell temperature again exceeds T.sub.1.
The method of controlling a power system according to the this invention can also include the steps of monitoring the concentration of electrolyte in the electrolyte solution and injecting additional electrolyte into the electrolyte solution when the concentration drops below a predetermined minimum. The method may further comprise the steps of monitoring the temperature of the electrolyte circulating in a circulatory system that includes a heat exchanger with a cooling fan, and turning on the cooling fan when the temperature of the electrolyte solution exceeds a first predetermined temperature and turning off the cooling fan when the temperature of the electrolyte solution is less than a second predetermined temperature.
The metal-air cell of this invention provides easy access to the anode in the pouch cathode so that the cell can be quickly and conveniently replenished with a replacement anode. The flexibility of the pouch cathode allows the cell to be compressed to maintain the precise anode-cathode spacing determined by the spacers, maintaining the cell's optimum performance. The variable inter-cell spacing in the stack provides for optimum circulation of depolarizing and cooling air between the cells maximizing their individual performance.
The power system optimizes performance of the cells. Monitoring of the concentration of the electrolyte in the electrolyte solution, and replenishment of the electrolyte, optimizes cell performance. Use of a sump and/or a filter to remove solids and reaction products from the circulating electrolyte solution maintains the quality of the electrolyte solution, maximizing performance of the cells. The use of a controller and a temperature sensor to either circulate the electrolyte solution through a heat exchanger, or to control the heat exchanger fan, helps to maintain the electrolyte solution at optimum temperature to maximize performance of the cells.
The start up controller provides quick start up of the power system when it is turned on when the system is cold. The pulse-on start up sequence in which the electrolyte is circulated a first predetermined period and retained in the cells for a second predetermined time more quickly brings the electrolyte solution to the optimum operating temperature. The controller also provides economical shut-down when the system is turned off. The controller initially provides a reverse bias voltage to the stack for a predetermined period, to retard further consumption of the anode, while retaining the system in a ready state for reactivation. After the predetermined period, the controller causes the circulatory system to drain the electrolyte solution from the cells to protect the anodes.
The method of controlling a metal-air cell power system of this invention provides for rapid start up, quickly bringing the electrolyte solution to optimum operating temperature. The method also maintains the electrolyte concentration, purity, and temperature of the electrolyte solution, optimizing performance of the power system. Finally, the method provides for efficient shut-down, providing a dwell period during which the anodes are protected from further consumption but the cells are ready for immediate use, and a shut down period in which the electrolyte is drained from the cells to protect the anodes from further consumption.
System start-up can be facilitated by the application of a surge current at turn-on to both warm up the electrochemical cell and to help de-passivation of the anodes within the cells.
These and other features and advantages will be in part apparent and in part pointed out hereinafter.